Last week, they announced, that it came from an infected external disk drive used to upload the flight programming.
The network itself is severed from the Internet and other systems, but they still needed to get the data onto the systems and the drive they used was infected. They announced last week, that the machines would now also be part of the no USB drive security level - although how they will get updates onto the machines is questionable, if they have no network and can't use external media...
More worrying is that they are using Windows to control the drones! Still, it could be worse, they could be using OS X and getting the beachball at critical moments!
